"Sending of password did not succeed"

I am using Thunderbird 3 on Win 7. I have 3 email accounts, the main one works fine.I have re-entered the details for the other two with my password and now when I first open the programme I get the error messageSending of password did not succeed, mail server mail.plus.net responded:Login failedI don't know if it is that but my Username is surname and for the Username on the other 2 accounts I have entered surname+john and surname+billI can't think what else it could be.